1. Technical Field
The present invention relates to a radio communication system for supplying operation power and a data signal by an electromagnetic wave (electric wave) from another radio device to a radio terminal device such as a RF-ID (Radio Frequency IDentification) tag system, a non contact type IC card system, and the like. In particular, the present invention relates to a radio communication system or the like which reduces electric power consumption in a radio terminal system when a radio terminal device is equipped with an indicator consuming large electric power.
2. Related Art
Development of a RF-ID device has been advanced as a substitution of a bar code and the like. Some of the RF-ID device incorporates an electric battery and some does not. The RF-ID device which does not incorporate an electric battery is activated by receiving supply of electric power by an electromagnetic wave such as a carrier wave from outside to transmit and receive data to and from an outer device. The typical application of the RF-ID device is a tag for merchandise management. Alternatively, an information display terminal and the like to which the RF-ID technology is applied has been considered. However, when an indicator is attached to the RF-ID device which does not incorporate an electric battery, supply of electric power to the RF-ID device is stopped when the RF-ID device is gone out from the range within which an electric wave from a radio device (information post) for providing a display information is reached or radio communication is finished. In this case, it is impossible to continue display. Consequently, for example, in the RF-ID device described in JP-A-2005-165814, using of so called an electronic paper display (EPD) having a property (display memory property) for keeping a display state even when power source is turned off as an indicator is proposed. A driving voltage about, for example, 10 volt is required for the EPD. In the RF-ID device, the driving power of the EPD indicator is considerably larger than that of another circuit, so that devices have been made in order to obtain a required indicator driving power and device driving power by, for example, providing a power receiving antenna for driving the indicator, driving a communication unit and a display unit in a time division manner, providing a large scale stepwise booster circuit.
As described above, the RF-ID device receives power supply from an external information post or the like by an electromagnetic coupling (electric wave) to be operated. Accordingly, operation power supply to the RF-ID device can be increased by increasing the radiation power of the information post.
However, the radio emission level should be reduced as much as possible as the radio emission may cause noise, communication failure, and the like. Further, there exists output regulation by the Radio Law. Further, not only the reduction of the electric power consumption of the RF-ID device, but also the reduction of the electric power consumption of the whole of the RF-ID device system is preferable.